1. Field of the Invention
This invention relates to the control of an end effector of an industrial robot that moves continuously along a predetermined path. More particularly, this invention pertains to the definition of the path along which the end effector moves.
2. Background of the Invention
Industrial robots perform tracking tasks by continually moving a tool or end effector held at the end of a robot arm. Control strategies have been developed, such as point-to-point and continuous path techniques, to guide the movement of the robot tool along the path. The point-to-point technique causes the tool to move preferably in straight lines between a sequence of points that define a path determined by robot joint angle interpolation. The continuous path technique for controlling the movement of the tool requires storing in a computer memory the coordinates of a very large number of closely spaced points that define the path. Another technique to control the path generates a sequence of points on the path in task space and then transforms the points to robot joint angles for execution. Programming the tracking task is difficult and editing a defined path becomes nearly impossible.
The curvilinear path interpolation procedure for controlling movement of the robot tool along a path generally requires a polynomial fit of a sequence of discrete points in a task space to produce a dynamically smooth and efficient trajectory. Such paths are of little relevance where the robot is to perform a tracking function because the correct path is uniquely defined by the geometry of the workpiece.
In the related field of numerically controlled machines, circular, elliptical, parabolic curvilinear interpolation has been practiced for some time. The problem of robot tracking is different from numeric control techniques, however, because here the programmer usually does not know the analytical description of the path, such as its radius, or the location of the center of a circular arc, or the major or minor axes of an ellipse.
The multi-circular curvilinear interpolation method of this invention defines a sufficiently accurate tracking path in cartesian space by using concatenated segments of circular arcs. In this way, a curve of continuously varying curvature is fitted by curves of stepwise changing curvature, whereby portions of the required path may be made easily to conform to circular or straight paths.